The small arteries and arterioles are the effector tissue which ultimately controls the total peripheral resistance to blood flow, and they are major determinants of blood pressure. These vessels and the smooth muscle cells in their walls are therefore intimately involved in the etiology and maintenance of essential hypertension. This proposal is concerned with the measurement of the structural and functional differences between such vessels obtained from the mesentery of the spontaneously hypertensive rat (SHR) and its normotensive control, the Wistar-Kyoto rat (WKY). These studies encompass wall morphology, contractile properties of isolated arterial segments, and the in vivo responsiveness of arteries of 25-250 microns lumen diameter. The research includes: 1) The determination of the effects of anti-hypertensive drug therapy on smooth muscle cell size and number and on cell contracility. 2) The measurement of the norepinephrine dose-response characteristics of the vascular smooth muscle cells. 3) The pressure-diameter and mechanical properties of excised 75-250 micron arteries. 4) The use of micropressure techniques to define the pressure-diameter relationships of 25-250 micron activated and relaxed vessels in anesthetized rats. 5) The correlation of wall morphology between the arteriole and the small artery. This coordinated study of a single resistance bed from a single animal model of hypertension will provide new information and insights into the diverse abnormalities affecting the cardiovascular system and will have relevance to human essential hypertension.